1. Field of the Invention
This invention relates generally to ozone generators, and more specifically to an improved method for controlling the output of such ozone generators.
2. Description of the Prior Art
Ozone is a very powerful gaseous reactant, and its use has been well established for many years in a wide range of industrial applications. Recently its value in all types of water purification applications has been coming to the fore because of its ability to act as a powerful oxidant, microflocculant and disinfectant without producing toxic by-products. Various means have been used to generate ozone, including AC or DC high-voltage corona discharge, UV light, and microwaves. All of these ozone generators produce ozone "on the spot" (ozone is generally not stored) from a flow of either air or oxygen feed gas, and they all utilize a power supply to provide electrical power. Any variation in gas flow, temperature, or pressure affects the concentration and quantity of ozone produced, and yet the maintenance of a constant ozone output is highly desirable. The rate of gas flow and the temperature (and in negative pressure applications, the pressure) are variables which are difficult and expensive to stabilize.
In the prior art a constant ozone output has been accomplished by feedback control to the power supply (to adjust power level) either from a monitor of gaseous ozone in the gas line downstream from the ozone generator, or from a monitor of dissolved ozone in the water into which the ozone has been injected. Such ozone monitors cost in excess of $5,000, and their feedback control loops can have time lag and oscillation problems. In addition, if a low ozone output were due to a high temperature, the feedback response of raising power input would increase the temperature even further, making things worse. Another possible control method would be feedback of a signal from a gas flow sensor to an automatic regulating valve to maintain a constant gas flow, but automatic regulating valves are prohibitively expensive for most applications, and cannot protect the ozone generator in case of abnormally low flows.
The most practical and widely used method of generating ozone is a high voltage AC corona discharge, where dry air or oxygen flows through a narrow gap or annulus bordered on one side by a metal electrode and on the other by a dielectric electrode surfaced with an electrical conductor. An alternating current of high voltage is connected across the electrodes, producing a high voltage field across the gap which results in the corona discharge. This discharge, which is also known as a "silent discharge" or "cold plasma discharge", converts a percentage of the gas to ozone. The high voltage necessary to drive the corona discharge is produced by a power supply which includes a transformer capable of boosting low voltage AC to a high voltage, usually between 5 and 25 kilovolts. The amount of power drawn from the power supply by the ozone generator is a function of the voltage, the frequency or pulse repetition rate, the gas flow rate, the temperature, and the pressure. The gas flow rate, temperature, and pressure also affect the efficiency of ozone production. The maximum amount of ozone that can be generated depends, in addition to the above factors, on corona cell geometry, the properties of the dielectric used, and the efficiency of cooling. At least two factors limit the highest ozone concentrations efficiently attainable: at higher temperatures the rate of ozone destruction increases; and at higher concentrations the rate of ozone formation decreases, not only becoming self-limiting, with the limit being lowered by higher temperatures, but also, under conditions of low gas flow and high electrical current rather suddenly reaching a point of over-ionization and ozone destruction.
It is desirable to generate as high a concentration of ozone as is efficiently possible because ozone has a rather low solubility in water, and because low ozone concentrations require the use of more dry air or oxygen feed gas and have more off-gassing problems than high concentrations. The generation of high concentrations requires a relatively low gas flow, more power usage per gram generated, and results in fewer grams/hour than the generation of lower concentrations, which uses a higher gas flow, more (but not commensurately more) power to the ozone generator, less power usage/gram (i.e., higher efficiency), and results in considerably larger total quantity (grams/unit time) of ozone.
Furthermore, it would be very desirable to be able to employ a single ozone generator for a variety of different uses or users simultaneously, each user requiring or drawing a different gas flow and going on and off independently of each other, such as in a private installation with a swimming pool and a spa, or in a commercial health club with a swimming pool, a spa, and a baby pool. Known multiple use installations presently utilize a separate high voltage transformer and ozone generator for each end user.
For reliable and long-lived operation of corona discharge-type ozone generators, a means should be provided to insure that over-ionization of the corona cannot occur. Over-ionization results when the ratio of power drawn by the corona to gas flow rate through the corona (power/flow rate) rises above a certain critical threshold ratio. This critical ratio falls (the less forgiving direction) as the temperature of the corona cell increases. Thus either a rise in ambient temperature or a drop in gas flow could cause this threshold to be crossed. Reducing the gas flow decreases the ability of the flow to cool the corona, thereby allowing the temperature to increase, and the slower flow also allows over-ionization to begin to occur because of the longer residence time of the ions in the corona cell. Over-ionization itself causes the temperature to rise because it causes the corona to draw more current from the power supply. Both of these temperature factors lower the critical threshold ratio, resulting in a vicious cycle which creates a condition of sudden overheating, a precipitous drop in ozone output, and a conversion of the corona from many tiny microdischarges to larger and hotter discharges which cause "hot-spot" erosion of metal electrodes and contribute to the dielectric breakdown of dielectric electrodes. Over-ionization also produces a very high level of electromagnetic radiation which wreaks havoc with the power supply and its controls.
Known prior art systems have dealt with the over-ionization problem by shutting down the power supply completely if the gas flow drops below a fixed set-point. In the prior art it has been necessary to select the value of this set-point to be above the critical value at which over-ionization of the corona would begin to occur under worst case conditions of temperature and pressure, with the ozone generator running at its fixed power level. In addition, a margin of safety must be included when choosing the fixed power level in order to take into consideration other worst case conditions including flow rate reducing factors such as gas filters which become clogged between servicings, a water filter whose back-pressure affects the suction of a venturi and is in need of back-washing, and the line voltage to the pump dropping as low as it typically fluctuates. The use of a fixed power level selected for worst case conditions means that some sacrifice in ozone concentration or efficiency is being made whenever better conditions exist, which is most of the time. Furthermore, when operating at a low ambient temperature, the flow rate at which over-ionization of the corona occurs is far less than it is at high ambient temperature, and thus, since gas flow sensing in prior art has been limited to a go/no-go signal which disables the power supply if the gas flow rate falls below the fixed set-point value, many of the shutdowns which would occur due to dips in the gas flow rate are not really necessary. Also, a go/no-go control does not work in multiple use situations, where it would be desirable to keep the ozone generator functioning efficiently over a wide range of gas flows, and yet be protected from over-ionization.
Therefore in most ozone applications it would be of great benefit to have an inexpensive method for automatically maintaining a constant ozone output (either concentration: percent ozone, %; or quantity: grams of ozone/hour, which is equal to the product of concentration and gas flow rate) which is generally independent of variations in the flow rate of the air or oxygen feed gas (and in some cases also independent of variations in temperature and or pressure. Furthermore, in multiple-use installations it would be extremely desirable to provide a means of automatically varying the power fed to the ozone generator so as to maintain a constant ozone output (concentration or quantity) independent of changes in flow rate due to different user systems going on and off. It would also be advantageous to be able to automatically provide, for each flow rate, the optimum power level for either maximum ozone concentration or maximum ozone generation efficiency (weight of ozone generated per Kilowatts used), or any compromise between these parameters.
Ozone generators can be operated at either positive or negative pressure. Either the feed gas source or the end usage of the ozone flow can cause variations in the flow rate. The flow resistance of feed gas preparation filters increases slowly with time as the filter pores fill up. In water purification applications, typical end use means of injecting ozone into water include venturis, static mixers, spargers, and turbines, all of which can and do vary in the amount of flow they either pull or allow, depending on the line voltage to a pump driving a venturi or to a turbine injector, the cleanliness of in-line water filters, the water levels and other back-pressure parameters. It is both difficult and expensive to use an automatic water control system to maintain a constant gas flow under all conditions. Although the gas flow rate itself might be held constant with a motorized regulating valve controlled by a signal from a flow rate sensor, this mechanical solution is prone to breakdown and would be prohibitively expensive for most applications. Particularly, the preferred installation utilizes a venturi and a negative pressure, which would require any such valve to be placed downstream of the ozone generator in order to avoid creating an excessively negative pressure within the generator itself, and thus the valve would have to be ozone-proof, which requires costly materials. Furthermore, in multiple-use installations, the control of gas flow would require separate ozone-proof gas flow sensors and ozone-proof automatic continuously adjustable valves for each user system branch. This would be extremely expensive and complex, and would not be able to vary the power to the ozone generator to maintain, for example, a constant ozone concentration if any of the component systems were to go off.
The purpose of the present invention is to provide a practical, inexpensive and automatic method of compensating for changes in gas flow and optionally also in temperature or pressure by controlling the power used by an ozone generator so that power corresponds to gas flow (and optionally temperature or pressure) so as to:
1) produce the maximum possible ozone concentration for each flow rate, or to maintain either a constant concentration or constant quantity of ozone output;
2) provide for independent multiple user operation; and
3) prevent over-ionization without necessitating shutdown.